Polyvinyl alcohol is one of the few crystalline water-soluble polymers. Utilizing its excellent water-solubility, film properties (such as strength, oil resistance, film-forming properties, and oxygen gas barrier properties), polyvinyl alcohol is widely used for emulsifiers, suspending agents, surfactants, fiber processing agents, various binders, paper processing agents, adhesives, films, and the like. Polyvinyl alcohol is used in the form of an aqueous solution except for special cases, where there may be difficulties in handling in some cases. For example, in preparation of the aqueous solution, there may be cases where polyvinyl alcohol remains as undissolved matter unless heating is performed at high temperature for a long time.
As a method for improving the water solubility of polyvinyl alcohol, a method of introducing an ionic group into polyvinyl alcohol is known. However, in the method of introducing an ionic group, a film to be formed tends to absorb moisture and may have low strength in some cases.
Patent Document 1 discloses a modified polyvinyl alcohol containing a hydroxyalkyl group having 2 to 20 carbon atoms in its side chain. It is described that an aqueous solution of the modified polyvinyl alcohol slightly foams despite its high surface activity. However, in the case of using the modified polyvinyl alcohol, although the water solubility is improved, film strength and gas barrier properties are insufficient. Thus, a further improvement has been desired.
Patent Document 2 discloses a composition containing an aqueous emulsion using a modified polyvinyl alcohol containing a monomer unit represented by formula (3) below as a dispersant, and use of the composition as an adhesive. The modified polyvinyl alcohol used in the composition is merely used as a dispersant in emulsion polymerization, and there is no description about the properties of the modified polyvinyl alcohol itself. Further, only the modified polyvinyl alcohol that contains a 3-methyl-3-buten-1-ol unit and that has a structure in which a 2-hydroxyethyl group is bonded to its main chain is used in Examples of Patent Document 2.

Patent Document 3 discloses a modified polyvinyl alcohol containing a 1,2-glycol component in its side chain, and it is described that an aqueous solution of the modified polyvinyl alcohol has excellent viscosity stability. However, in the case of using the modified polyvinyl alcohol, there has been a problem that film strength and gas barrier properties are reduced under high humidity conditions due to the influence of moisture absorption.
Patent Document 4 discloses a polyvinyl alcohol in which the content of 1,2-glycol bond in its main chain is increased by polymerization at a temperature higher than normal. It is described that an aqueous solution of the polyvinyl alcohol has excellent viscosity stability at low temperature. However, in the case of using the polyvinyl alcohol, there has been a problem that film strength and gas barrier properties are reduced under high humidity conditions due to the influence of moisture absorption.